The invention relates to a circuit for a telephone station which is main-supplied or remote-powered as necessary, and where a switching regulator is provided, which at its output supplies a constant supply voltage and at its input is bridged by a buffer capacitor.
Telephone stations are generally remote-powered by the telephone exchange via the subscriber line (a/b wire). In the case of telephone stations of luxury design (generally speaking telecommunications terminals) the numerous subscriber services and the display thereof increases power consumption so that power supply via the subscriber line is uneconomical. This applies particularly in the case of long connection distances. Therefore for telephone stations of this kind stationary power supply devices have been provided, to satisfy the higher power requirements of such stations.
However, power supply devices of this kind have the disadvantage that they are dependent upon the electrical mains. In the event of a power failure, telephone operation would be impossible. Therefore it is necessary in the case of a mains failure, to switch over to remote power, thereby disconnecting larger power consumers in the terminal. It is unfeasable for such a switchover to be manually carried out. A simple switchover circuit linking the two voltage sources to the statiobn via diodes is likewise inexpedient, since power supply via the power supply device should take precedence and therefore the power supply device output voltage must be higher than the DC voltage across the subscriber line. Since this DC voltage is limited to at most 72 V in the case of short connections, in this case the power supply device requires an increased contact protection and therefore becomes too expensive. On the other hand the conversion of a lower power supply device voltage into a higher DC voltage necessitates an additional DC voltage converter and increased space requirements in the terminal and an increased interference level is likely.
Therefore one object of the invention is to provide a simple circuit arrangement with which it is possible to effect an automatic switchover from mains supply to remote power in the event, for example, of a mains failure, which involves neither substantial space requirements, nor increased insulation, nor additional expense for parts.
This is achieved in that the first terminal of the buffer capacitor is connected via a first diode, which is biased in the flux direction, to the positive pole of a power supply device, is connected via a second diode, biased in the flux direction, and via the drain source path of a first transistor (MOS-transistor) to the positive pole of the remote feed, and is connected via the first diode and via two series-connected resistors to the negative poles of remote feed and power supply device and to its other terminal, that moreover the base of the first transistor is connected via the series arrangement of a third resistor, a first and a second Zener diode to the negative poles, and is connected via the third resistor and a fourth resistor to the positive pole of the remote feed, and that a second transistor is connected by its base to the common point of the series-connected resistors, is connected by its collector to the common point of the Zener diodes, and is connected by its emitter to the negative poles.
This has the advantage that is is easily possible in telephone stations as described above to switch over from the power supply device to remote power (for example if the mains fail) and vice versa economically and without the assistance of the operator.